Use custom docker image for CI

Signed-off-by: Thomas Citharel <tcit@tcit.fr>
4 jobs for master in 3 minutes and 22 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Check
passed check #583797

00:01:23

 
  Test
passed mix #583798

00:01:06

 
  Deploy
passed pages #583799

00:00:53

passed pages:deploy

00:00:00